BMW 7 Series: Functions

Steering Systems

Implementation of the Integrated Active Steering function has essentially been made possible by the new ICM system complex on the F01/F02.

The Servotronic function including valve control is also taken over by the ICM control unit. That steering control function is also influenced by the Driving Dynamics Control function.

Advantages of Integrated Active Steering:

• Extension of Active Steering (AL) by the addition of rear-wheel steering (HSR)
• Variable steering-gear ratio (steering angle amplification factor)
• Independent control of rear-wheel steering angle (steer by wire)
• Servotronic
• Handling stabilization functions
• Reduction of braking distance under split surface braking conditions.

Supply of Signals

Signals from external sensors

The ICM control unit reads the following signals that are essential to the Integrated Active Steering from external sensors:

• Four wheel-speed signals sent via Flexray by the DSC
• Steering angle sent via Flexray by steering column switch cluster
• Status of AL and HSR actuators transmitted via Flexray.

However, because the rear-wheels are steerable, the steering angle of the front wheels alone is not definitive for dynamic handling control purposes. Therefore, the ICM control unit also takes the steering angle of the rear wheels into account. Ultimately, the effective steering angle is calculated from the two steering angles (front and rear wheels). The effective steering angle indicates the angle to which the front wheels would have to be turned to bring about the same vehicle response without steerable rear wheels. That variable is the easiest for all vehicle systems to use to analyze the steering action.

Control and Modulation of Steering

Both the basic steering system and the optional Integrated Active Steering on the F01/F02 incorporate the Servotronic function. That speed-sensitive power assistance function is effected by way of the Servotronic valve on the steering gear.

The Servotronic valve is always controlled by the ICM control unit regardless of the equipment options fitted.

Accordingly, the Servotronic function algorithm is stored on the ICM control unit.

Similarly regardless of equipment options, the steering system also always incorporates a proportional control valve which is controlled by the ICM control unit. With the aid of that valve, the power steering pump's volumetric flow rate can be electronically adjusted. For that reason it is also referred to as the "electronic volumetric flow control" valve (EVV valve).

That valve too is controlled by the ICM control unit. Depending on the degree of power assistance demanded at the time, the volumetric flow rate delivered by the power steering pump is split between the steering valve and a bypass circuit. The ratio of that split can be infinitely varied. The less power assistance is required, the more hydraulic fluid is diverted into the bypass circuit. As the hydraulic fluid does not have to do any work in the bypass circuit, less power is required to drive the power steering pump. Consequently, the proportional control valve helps to reduce fuel consumption and CO2 emissions.

Fig. 85: Input/Output Signal Diagram - Control Of Steering By ICM

INDEX REFERENCE CHART

1. Steering column switch cluster
2. Wheel speed sensor
3. Dynamic stability control
4. Electronic volumetric flow control (EVV) valve
5. Servotronic valve
6. Integrated Chassis Management
7. "Steering control" function on ICM

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